(1) Field of the Invention
The present invention relates to a device for preventing noise generated from digital electronic apparatus from interfering with audio equipment and, more particularly, the invention relates to a noise preventing device for electronic apparatus of an automotive vehicle and to preventing interfering with or disturbing audio equipment thereon, such as a radio. The invention also pertains to a method of manufacturing the device.
(2) Description of the Prior Art
Recently digital electronics and microcomputers have been used in various types of consumer products, e.g., clocks, timers, and electric home appliances.
On the other hand, in the field of automotive vehicles electronic technology has been put into practice for controlling internal combustion engines and various types of electric appliances mounted in the automotive vehicle using digital wired logic circuitry or microcomputers.
In such a microcomputer-based electronic control system for the automotive vehicle, a metallic casing is provided for shielding the power supply and digital circuitry including the microprocessor, clock oscillator, memory unit, input/output interface and control circuits connected between the microprocessor and I/O interface. Such a metallic casing is installed below the instrument panel or seat where environmental conditions, e.g., temperature and humidity are relatively favorable within the vehicle and where input/output signal lines externally wired from the digital circuitry serve to transmit input signals from various types of sensors for calculating control parameters and output signals from the I/O interface for controlling the internal combustion engine, various electric appliances, and actuators, etc.
The frequency of the input/output signals to be transmitted through the input/output signal lines ranges from zero to several tens of kilohertz (kHz). In addition, for these input/output signal lines and power supply lines vinyl-coated wires are usually used and bundled with a set of harnesses to other electrical appliances. Because the wiring of the input/output signal lines is carried out in such a way as described above, the input/output signal lines are closely coupled to a set of harnesses for connecting radio equipment to the antenna. As a result, these input/output signal lines are coupled to the antenna harness by mutual inductance and electromagnetic radiation in the high-frequency range. Furthermore, since the vehicle battery is used as a common DC power supply of the digital circuitry and other electrical appliances and a vehicle body is used as a common ground of these pieces of vehicle equipment it is apparent that these pieces of vehicle equipment are conductively coupled to each other.
On the other hand, rectangular clock pulses with a frequency between several hundred kilohertz and several kilohertz are used for synchronization control signals of the digital electronic equipment, e.g., microcomputer. These signals comprise a basic wave with an amplitude of several volts and high harmonic components thereof. Each frequency of the high harmonic components is distributed over a wide range, i.e., from an AM band to an FM or TV band. Therefore, such a rectangular clock pulse may be deemed to be a kind of noise source. A leakage of high-frequency noise components of the clock pulse from the digital circuitry into the input/output signal lines and power supply lines and a direct and indirect coupling of the input/output signal lines and power supply lines to the radio harness and antenna cause the signal-to-noise ratio of the radio equipment to be reduced and causes beat interference in the radio equipment. Consequently, the quality of reception of an audio signal is reduced. A filter element for bypassing high-frequency waves is inserted as a noise interference device between each input/output signal line and the power supply line connected to all digital equipment and furthermore electrically connected to a ground conductor patterned on a printed circuit board which is electrically connected to a vehicle body or metallic casing of the equipment.
However, since such an earth conductor is needed on a lower surface of the printed circuit board (lower surface means one surface of the printed circuit board where metal foil patterns are regularly formed and soldered) it is difficult to connect a connector pin to a printed power supply line, power supply line, and ground line on its lower surface. If such a filter element is assembled on a separate printed circuit board, the order of assembly is facilitated and the ground conductor may cover a wide space so as to have a filtering effect on noise propagation. However, this arrangement reduces the effect of inserting the filters when many lines are required for connecting the input/output signal lines to the filter and requires extra spaces for these lines and filters.